How do institutions like banks do RSA with big primes?@CodesInChaos Well, lets say the bank's computer has to calculate (a^b) mod n. Where a is some number, could be 065066067, which is ASCII for ABC and b is their 255 digit long decryption key, lets just say it's 10^254, that means, they have to do 065066067*065066067*065066067... 10^254 times, if their computer's processor can do 5 billion multiplications per second, then it'd take (10^254)/(10^9) seconds, which is 3.2*10^237 years